Swapping letters

n = int(input())

forbidden = {}
for _ in range(n):
    a, b = input().split()
    if a not in forbidden:
        forbidden[a] = {b}
    else:
        forbidden[a].add(b)

    if b not in forbidden:
        forbidden[b] = {a}
    else:
        forbidden[a].add(a)

x = input()
y = input()

if len(x) != len(y):
    print('NO')
else:
    for i in range(len(x)):
        if x[i] == y[i]:
            continue
        ind = i + x[i+1:].index(y[i]) + 1
        for j in range(i, ind):
            if x[j] in forbidden[y[i]]:
                break
        else:
            x = x[:i] + x[ind] + x[i:ind] + x[ind + 1:]
            continue
        print('NO')
        break
    else:
        print('YES')

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 27, in <module>
    if x[j...

Test 5

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 27, in <module>
    if x[j...

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 27, in <module>
    if x[j...

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED